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Abstract.Intraplate earthquake zones in the back arc of NE Japan were imaged by wide-band magnetotelluric

This paper reports on a magnetotelluric (MT) survey across the central Mariana subduction system, providing a comprehensive electrical resistivity image of the upper mantle to address issues of mantle dynamics in the mantle wedge and beneath the slow back‐arc spreading ridge. After calculation of MT response functions and their correction for topographic distortion, two‐dimensional electrical resistivity structures were generated using an inversion algorithm with a smoothness constraint and with additional restrictions imposed by the subducting slab. The resultant isotropic electrical resistivity structure contains several key features. There is an uppermost resistive layer with a thickness of up to 150 km beneath the Pacific Ocean Basin, 80–100 km beneath the Mariana Trough, and 60 km beneath the Parece Vela Basin along with a conductive mantle beneath the resistive layer. A resistive region down to 60 km depth and a conductive region at greater depth are inferred beneath the volcanic arc in the mantle wedge. There is no evidence for a conductive feature beneath the back‐arc spreading center. Sensitivity tests were applied to these features through inversion of synthetic data. The uppermost resistive layer is the cool, dry residual from the plate accretion process. Its thickness beneath the Pacific Ocean Basin is controlled mainly by temperature, whereas the roughly constant thickness beneath the Mariana Trough and beneath the Parece Vela Basin regardless of seafloor age is controlled by composition. The conductive mantle beneath the uppermost resistive layer requires hydration of olivine and/or melting of the mantle. The resistive region beneath the volcanic arc down to 60 km suggests that fluids such as melt or free water are not well connected or are highly three‐dimensional and of limited size. In contrast, the conductive region beneath the volcanic arc below 60 km depth reflects melting and hydration driven by water release from the subducting slab. The resistive region beneath the back‐arc spreading center can be explained by dry mantle with typical temperatures, suggesting that any melt present is either poorly connected or distributed discontinuously along the strike of the ridge. Evidence for electrical anisotropy in the central Mariana upper mantle is weak.

A seafloor geomagnetic observatory in the northwest Pacific detected clear electromagnetic (EM) variations associated with tsunami passage from two earthquakes that occurred along the Kuril Trench. Previous seismological analyses indicated that the M8.3 earthquake on 15 November 2006 was an underthrust type on the landward slope of the trench, while the M8.1 earthquake on 13 January 2007 was a normal fault type on the seaward side. The EM measurements enabled precise monitoring of the tsunami propagation direction as well as particle motion of the seawater. The estimated horizontal water velocity differs significantly for the 2006 and 2007 tsunamis, in terms of initial motion and dispersive characters, being consistent with the hydrodynamic simulation results of the tsunamis. Namely, the tsunami‐induced horizontal geomagnetic components showed opposite signs for the rise and retreat waves as expected from the “electric current wall hypothesis.” The dispersion effect is more remarkable in the 2007 event with a smaller source region of its tsunamigenic earthquake. The 2007 tsunamis, therefore, tend to violate the long‐wave approximation. The Boussinesq approximation was required to reproduce the dispersive character of the 2007 event in our numerical simulation. In terms of tsunami forecast, an important advantage of EM sensors over conventional tsunami sensors, such as seafloor pressure gauges, is their capability of vector measurements: in addition to their ability to monitor particle motions, the first peak of the downward magnetic component always precedes the tsunami peak, suggesting a significant improvement in global tsunami warning systems if vector EM sensors are integrated into the existing systems.

We performed 3‐D time domain simulation of geomagnetically induced currents (GICs) flowing in the Japanese 500‐kV power grid. The three‐dimensional distribution of the geomagnetically induced electric field (GIE) was calculated by using the finite difference time domain method with a three‐dimensional electrical conductivity model constructed from a global relief model and a global map of sediment thickness. First, we imposed a uniform sheet current at 100‐km altitude with a sinusoidal perturbation to illuminate the influence of the structured ground conductivity on GIE and GIC. The simulation result shows that GIE exhibits localized, uneven distribution that can be attributed to charge accumulation due to the inhomogeneous conductivity below the Earth's surface. The charge accumulation becomes large when the conductivity gradient vector is parallel or antiparallel to the incident electric field. For given GIE, we calculated the GICs flowing in a simplified 500‐kV power grid network in Japan. The influence of the inhomogeneous ground conductivity on GIC appears to depend on a combination of the location of substations and the direction of the source current. Uneven distribution of the power grid system gives rise to intensification of the GICs flowing in remote areas where substations/power plants are distributed sparsely. Second, we imposed the sheet current with its intensity inferred from the ground magnetic disturbance for the magnetic storm of 27 May 2017. We compared the calculated GICs with the observed ones at substations around Tokyo and found a certain agreement when the uneven distribution of GIE is incorporated with the simulation.

On 29 June 2015, a small phreatic eruption occurred at Hakone volcano, Central Japan, forming several vents in the Owakudani geothermal area on the northern slope of the central cones. Intense earthquake swarm activity and geodetic signals corresponding to the 2015 eruption were also observed within the Hakone caldera. To complement these observations and to characterise the shallow resistivity structure of Hakone caldera, we carried out a threedimensional inversion of magnetotelluric measurement data acquired at 64 sites across the region. We utilised an unstructured tetrahedral mesh for the inversion code of the edge-based finite element method to account for the steep topography of the region during the inversion process. The main features of the best-fit three-dimensional model are a bell-shaped conductor, the bottom of which shows good agreement with the upper limit of seismicity, beneath the central cones and the Owakudani geothermal area, and several buried bowl-shaped conductive zones beneath the Gora and Kojiri areas. We infer that the main bell-shaped conductor represents a hydrothermally altered zone that acts as a cap or seal to resist the upwelling of volcanic fluids. Enhanced volcanic activity may cause volcanic fluids to pass through the resistive body surrounded by the altered zone and thus promote brittle failure within the resistive body. The overlapping locations of the bowl-shaped conductors, the buried caldera structures and the presence of sodium-chloride-rich hot springs indicate that the conductors represent porous media saturated by highsalinity hot spring waters. The linear clusters of earthquake swarms beneath the Kojiri area may indicate several weak zones that formed due to these structural contrasts.

We obtained an electrical transect image of the Niigata‐Kobe Tectonic Zone (NKTZ). Several major active faults are located in this zone of concentrated deformation. The main features of the final two‐dimensional model are a thick resistive block in the upper crust, with a thinned‐out portion beneath the Atotsugawa Fault, and a strong conductor in the lower crust that intrudes upward into the upper resistor. The upper crustal resistive zone corresponds well to the spatiality of the NKTZ, and relatively conductive zones sandwiching this resistor may contribute to observed changes in displacement rates. The overlapping locations of the conductor and the low‐velocity body in the lower crust indicate that the conductor represents a zone that was weakened by fluids. Given that microearthquakes are localized in the regions between the resistive and conductive zones, we suggest that the distribution of earthquakes is influenced by intrusions of fluid derived from the conductor.

Sagami Bay is an active tectonic area in Japan. In 1993, a real-time deep sea floor observatory was deployed at 1,175 m depth about 7 km off Hatsushima Island, Sagami Bay to monitor seismic activities and other geophysical phenomena. Video cameras monitored biological activities associated with tectonic activities. The observation system was renovated completely in 2000. An ocean bottom electromagnetic meter (OBEM), an ocean bottom differential pressure gauge (DPG) system, and an ocean bottom gravity meter (OBG) were installed January 2005; operations began in February of that year. An earthquake (M5.4) in April 2006, generated a submarine landslide that reached the Hatsushima Observatory, moving some sensors. The video camera took movies of mudflows; OBEM and other sensors detected distinctive changes occurring with the mudflow. Although the DPG and OBG were recovered in January 2008, the OBEM continues to obtain data.